This is pretty simple, I've tried to do the bare minimum to get things working, rather than make things too complex, but it solves my initial use-case, and making future changes with generics should be a simple additive process when new use-cases are found.

We basically pull the type params out of the generics object (i.e. not the lifetimes, and not any const parameters either), and, if there are any, create a where-clause of T: ::ramhorns::Content for each type param (i.e. T) that has been declared.

There are probably improvements that could be made, such as not adding the type bound if the ramhorns(skip) attribute is used for that field, or copying over other type bounds that exist, but I figure they can be added later if they are likely to actually be useful to someone.

Closes #38